This invention relates to titanium dioxide particles having a passivating coating.
Titanium dioxide is widely used as a filler and pigment in paints and other coatings, in many polymer systems, in paper, and in other applications. Titanium dioxide has a characteristic bright, white color, which makes it a pigment of choice in many applications.
One problem with using titanium dioxide is that it catalyzes a large number of reactions, particularly among organic materials, and it also can participate in numerous other reactions. In some coating and polymer systems, titanium dioxide can catalyze unwanted depolymerization reactions or catalyze unwanted reactions between components of the system. This can lead to problems such as a deterioration of physical properties, or a loss of other desirable attributes.
It has been proposed to coat titanium dioxide with a passivating coating, which would, in effect, encapsulate the titanium dioxide and prevent other materials from coming into contact with titanium dioxide species. Alumina and/or silica coatings have been proposed, because they are optically transparent in the visible light spectrum and are not highly reactive or catalytic species.
There remains the problem of applying such a coating to titanium dioxide particles. Titanium dioxide filler particles and pigments are finely divided, usually having an average particle diameter in the range of 1 micron or less. The titanium dioxide particles can be as small as 10 nanometers. It is very difficult to coat such small particles uniformly, and without “gluing” the particles together to form large agglomerates. It is also desirable to provide a coating that is as thin as possible, to reduce raw material costs and preserve the characteristic bright white appearance of the underlying titanium dioxide.
U.S. Published Patent Application No. 2004/0224087 describes coating TiO2 base particles with one or more SiO2 coating layers, using an atomic layer deposition process. In U.S. Pat. No. 6,713,177, it is proposed to use an atomic layer deposition process to coat titanium dioxide particles with a coating, such as a nitride, to reduce the photocatalyic properties of the titanium dioxide. The coating particles are described as being useful in certain coating applications. The atomic layer deposition process is described in these references as being capable of forming ultrathin, conformal coatings on substrate particles, without agglomerating the particles.
It has been discovered that when a silica or alumina coating is applied to titanium dioxide particles via an atomic layer deposition (ALD) process, the coated particles become colored. The coated particles exhibit a blue or green color, which is surprising considering that titanium dioxide, silica and alumina are all colorless materials. This coloration makes the coated particles useless in most applications in which a colorless filler is needed, or in which the bright white color of titania is needed.